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Different but (Probably) Equal

London - HYPOTHESIS: males and females are typically indistinguishable on the basis of their behaviors and intellectual abilities.

This is not true for elephants. Females have big vocabularies and hang out in herds; males tend to live in solitary splendor, and insofar as they speak at all, their conversation appears mostly to consist of elephant for "I'm in the mood, I'm in the mood"

The hypothesis is not true for zebra finches. Males sing elaborate songs. Females can't sing at all. A zebra finch opera would have to have males in all the singing roles.

And it's not true for green spoon worms. This animal, which lives on the sea floor, has one of the largest known size differences between male and female: the male is 200,000 times smaller. He spends his whole life in her reproductive tract, fertilizing eggs by regurgitating sperm through his mouth. He's so different from his mate that when he was first discovered by science, he was not recognized as being a green spoon worm; instead, he was thought to be a parasite.

Is it ridiculous to suppose that the hypothesis might not be true for humans either?

No. But it is not fashionable -- as Lawrence Summers, president of Harvard University, discovered when he suggested this month that greater intrinsic ability might be one reason that men are overrepresented at the top levels of fields involving math, science and engineering.

There are -- as the maladroit Mr. Summers should have known -- good reasons it's not fashionable. Beliefs that men are intrinsically better at this or that have repeatedly led to discrimination and prejudice, and then they've been proved to be nonsense. Women were thought not to be world-class musicians. But when American symphony orchestras introduced blind auditions in the 1970's -- the musician plays behind a screen so that his or her gender is invisible to those listening -- the number of women offered jobs in professional orchestras increased.

Similarly, in science, studies of the ways that grant applications are evaluated have shown that women are more likely to get financing when those reading the applications do not know the sex of the applicant. In other words, there's still plenty of work to do to level the playing field; there's no reason to suppose there's something inevitable about the status quo.

All the same, it seems a shame if we can't even voice the question. Sex differences are fascinating -- and entirely unlike the other biological differences that distinguish other groups of living things (like populations and species). Sex differences never arise in isolation, with females evolving on a mountaintop, say, and males evolving in a cave. Instead, most genes -- and in some species, all genes -- spend equal time in each sex. Many sex differences are not, therefore, the result of his having one gene while she has another. Rather, they are attributable to the way particular genes behave when they find themselves in him instead of her.

The magnificent difference between male and female green spoon worms, for example, has nothing to do with their having different genes: each green spoon worm larva could go either way. Which sex it becomes depends on whether it meets a female during its first three weeks of life. If it meets a female, it becomes male and prepares to regurgitate; if it doesn't, it becomes female and settles into a crack on the sea floor.

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What's more, the fact that most genes occur in both males and females can generate interesting sexual tensions. In male fruit flies, for instance, variants of genes that confer particular success -- which on Mother Nature's abacus is the number of descendants you have -- tend to be detrimental when they occur in females, and vice versa. Worse: the bigger the advantage in one sex, the more detrimental those genes are in the other. This means that, at least for fruit flies, the same genes that make a male a Don Juan would also turn a female into a wallflower; conversely, the genes that make a female a knockout babe would produce a clumsy fellow with the sex appeal of a cake tin.

But why do sex differences appear at all? They appear when the secret of success differs for males and females: the more divergent the paths to success, the more extreme the physiological differences. Peacocks have huge tails and strut about because peahens prefer males with big tails. Bull elephant seals grow to five times the mass of females because big males are better at monopolizing the beaches where the females haul out to have sex and give birth.

Meanwhile, the crow-like jackdaw has (as far as we can tell) no obvious sex differences and appears to lead a life of devoted monogamy. Here, what works for him also seems to work for her, though the female is more likely to sit on the eggs. So by studying the differences -- and similarities -- among men and women, we can potentially learn about the forces that have shaped us in the past.

And I think the news is good. We're not like green spoon worms or elephant seals, with males and females so different that aspiring to an egalitarian society would be ludicrous. And though we may not be jackdaws either -- men and women tend to look different, though even here there's overlap -- it's obvious that where there are intellectual differences, they are so slight they cannot be prejudged.

The interesting questions are, is there an average intrinsic difference? And how extensive is the variation? I would love to know if the averages are the same but the underlying variation is different -- with members of one sex tending to be either superb or dreadful at particular sorts of thinking while members of the other are pretty good but rarely exceptional.

Curiously, such a result could arise even if the forces shaping men and women have been identical. In some animals -- humans and fruit flies come to mind -- males have an X chromosome and a Y chromosome while females have two X's. In females, then, extreme effects of genes on one X chromosome can be offset by the genes on the other. But in males, there's no hiding your X. In birds and butterflies, though, it's the other way around: females have a Z chromosome and a W chromosome, and males snooze along with two Z's.

The science of sex differences, even in fruit flies and toads, is a ferociously complex subject. It's also famously fraught, given its malignant history. In fact, there was a time not so long ago when I would have balked at the whole enterprise: the idea there might be intrinsic cognitive differences between men and women was one I found insulting. But science is a great persuader. The jackdaws and spoon worms have forced me to change my mind. Now I'm keen to know what sets men and women apart -- and no longer afraid of what we may find.

Op-Ed Contributor Olivia Judson, an evolutionary biologist at Imperial College in London, is the author of "Dr. Tatiana's Sex Advice to All Creation: The Definitive Guide to the Evolutionary Biology of Sex."

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A version of this op-ed appears in print on January 23, 2005, on Page 4004017 of the National edition with the headline: Different but (Probably) Equal. Today's Paper|Subscribe